Paper tissue, also called cellulose wadding, is an absorbent paper of low specific surface weight between 15 and 35 g/m2 which may be manufactured to offer some stretching capacity. A conventional procedure adds glue to a still wet sheet and applies the sheet against a drying cylinder and then detaches it from the cylinder by a scraper blade in order to imprint crepe corrugations. Thereupon, the sheet's properties, such as softness, flexibility, absorption, thickness and appearance, may be improved or at least be modified using known embossing procedures.
Embossing is carried out either on a highly moist paper, that is in the wet portion of a papermaking machine, or on paper of low moisture, that is when being transformed in its dry mode. The present invention concerns embossing low moisture paper. Most commonly an embossing pattern consists of protrusions or bosses in the form of cross-sectionally square pyramids or cross-sectionally round or oval frustra of cones, which are distributed regularly or otherwise on the sheet's surface. It is noted that a protrusion on one side of the sheet corresponds to a cavity on the other side.
In a first technique, embossing tissue paper is carried out using a generally rigid cylinder fitted at its surface with embossing tips. The sheet is applied against the cylinder and compressed by another cylinder fitted with deforming cladding, such as rubber. In this manner the sheet is made to match the first cylinder's topography. For a given embossing pattern, the deformation is more or less pronounced depending on the parameters that were selected, namely the rubber's flexibility, its deformability and its ability to match the engraved topography, and the embossing pressure.
In another technique, two cylinders are fitted at their respective surfaces with mutually complementary topographies, namely male and female.
The embossing tips are made by engraving the cylinder, for example, using a knurling tool which itself is engraved. Depending on the particular material used, the embossing tips also may be machined directly.
The embossing tips assume a generally cross-sectionally frustoconical circular or oval shape or a pyramidic, cross-sectionally square shape. The half angle at the apex of an inscribed cone is typically between 25° and 35°. The embossing tips' dimensions, their distribution, their density (their number per unit area), are selected as a function of the desired product performance.
Illustratively, if it is desired to impart a textile appearance to a sheet and to flexibilize it without particularly improving its absorptivity, very shallow heights are selected for the embossing tips, namely between 0.4 and 0.6 mm and they are distributed in a high density pattern (more than 30, even 80 embossing tips per cm2). On the other hand, as regards a paper used for wiping, in particular a paper towel, the sheet is substantially deformed in order to create air pockets between the plies following their assembly. The pockets appreciably enhance the absorption of the tissue paper. In that case, the tip height may be as large as 2.5 mm while the pattern density will be low, less than 10 tips/cm2. In this manner an embossing pattern consisting mostly of frustoconical embossing tips is characterized simultaneously by its height and by its density.
When the sheet is deformed beyond its elastic limit, part of the links between the paper fibers are always destroyed. Any embossing therefore entails a loss in paper tear-strength. On the other hand, the number of links having decreased, the sheet shall be more flexible.
When cellulose tissue paper is embossed using an embossing assembly of an engraved cylinder and a rubber cylinder, the sheet deformation, hence its thickness, depends on the pressure applied by the rubber cylinder. For a given pattern, the drop in tear strength therefore is related to the attained embossing thickness and becomes greater with it. This is especially the case at the maximum values. A maximum thickness corresponds to maximum loss of tear strength.